1. Field of the Invention
The present invention relates to an image heating apparatus which is applied to an image forming apparatus such as a copying machine or a printer, and more specifically an apparatus which utilizes an endless belt.
2. Related Background Art
For convenience, description will be made taking as an example an image heating apparatus (fixing apparatus) which is used in an image forming apparatus such as a copying machine or a printer for heating a toner image and fixing the toner image to a recording material.
A heating roller type image forming apparatus is widely used as a fixing apparatus which heats and fixes, as an image permanently fixed on a surface of a recording material, an unfixed image (toner image) of image data which is formed and born (carried) in a transfer mode or a direct mode on a recording material (transfer material sheet, Electrofax sheet, electrostatic recording paper, OHP sheet, printing paper, format paper or the like) by adequate image forming process means such as an electrophotographic process, an electrographic recording process or a magnetic recording process.
A heater contact film heating type apparatus has recently been put to practical use from viewpoints of quick start and energy saving. Furthermore, an electromagnetic induction heating type apparatus has been also proposed.
a) Heating Roller Type Fixing Apparatus
A heating roller type fixing apparatus has a basic configuration of a pair of pressure contact rollers consisting of a fixing roller (heating roller) and a pressurizing roller, and functions to introduce a recording material on which an unfixed toner image to be fixed is formed and carried into a fixing nip section (heating nip section) which is a mutual contact portion between the above described pair of rollers, sandwich and carry the recording material, and heat the unfixed toner image and fix the image to a surface of the recording material with heat of the fixing roller and a pressurizing force of the fixing nip section.
The fixing roller is generally composed of a hollow metal roller of aluminum as a base body (core metal) and a halogen lamp disposed in a hollow space as a heat source and is heated by heat generated by the halogen lamp, and power supply to the halogen lamp is controlled so as to maintain an outer circumferential surface of the fixing roller at a predetermined fixing temperature.
In case of a fixing apparatus of an image forming apparatus for forming an image in full colors which must have a capability to mix colors by sufficiently heating and melting four toner image layers at maximum and allows fixing to be improper due to insufficient heating down to an interface between a recording material and a toner layer in particular, an elastic layer of rubber is disposed so that a core metal of a fixing roller has a large heat capacity and a toner image is melted uniformly in a condition wrapped in an outer circumference of the core metal, and the toner image is heated by way of the elastic layer of rubber. Further, a certain fixing apparatus is configured to use a heat source disposed also in a pressurizing roller so that the pressurizing roller is also heated and kept at a controlled temperature.
b) Heater Contact Film Heating Type Fixing Apparatus
Heater contact film heating type fixing apparatuses are proposed, for example, by Japanese Patent Application Laid-Open No. 63-313182, No. 2-157878, No. 4-44075 and No. 4-204980.
Concretely speaking, a heater contact film heating type fixing apparatus is configured to form a fixing nip section generally by sandwiching a heat resistant film (fixing film or fixing belt) between a ceramic heater used as a heating body and a pressurizing roller used as a pressurizing member, introduce a recording material on which an unfixed toner image to be fixed is formed and carried between the fixing film and the pressurizing roller in the above described fixing nip section, and sandwich and carry the recording material together with the fixing film, thereby imparting heat of the ceramic heater to the recording material by way of the fixing film in the fixing nip section and fixing the unfixed toner image to a surface of the recording material with heat and a pressing force of the fixing nip section (portion).
This film heating type fixing apparatus permits composing an on-demand type apparatus using members having low heat capacities as the ceramic heater and the fixing film, and requires power supply to the ceramic heater used as a heat source to set it in a condition heated to a predetermined fixing temperature only during execution of image formation by an image forming apparatus, thereby providing merits to shorten a wait time from a power on time of the image forming apparatus till a condition ready for the execution of image formation (a quick start property) and remarkably lower power consumption in a standby condition (power saving property).
c) Electromagnetic Induction Heating Type Fixing Apparatus
Japanese Utility Model Application Laid-Open No. 51-109739 discloses an induction heating fixing apparatus which induces an electric current in a fixing roller with a magnetic flux, thereby heating the fixing roller with Joule heat. This fixing apparatus is capable of directly heating the fixing roller by utilizing generation of an induced current and providing a fixing process with an efficiency higher than that of the heating roller type fixing apparatus which uses a halogen lamp as a heat source.
A high efficiency fixing apparatus has been contrived by bringing an excitation coil nearer a fixing roller used as a heat generating body to obtain a fixing energy at a high density or concentrating a distribution of an alternating magnetic flux of an excitation coil in the vicinity of a fixing nip section.
FIG. 16 shows a schematic configuration of an example of electromagnetic induction heating type fixing apparatus which has an efficiency enhanced by concentrating a distribution of an alternating magnetic flux of an excitation coil on a fixing nip.
Reference numeral 10 denotes a fixing belt (fixing film) in a form of an endless (cylindrical) belt which has electromagnetic induction heat generating layers (a layer of an electrically conductive material, a layer of a magnetic material and a layer of resistor material), and is configured as a heating rotating body generating heat by electromagnetic induction.
Reference numeral 16 denotes a belt guide member (film guide member) in a form of a conduit which has a cross section nearly of a semicircle and the endless belt 10 is loosely fitted outside the belt guide member 16.
Reference numeral 15 denotes magnetic field producing means which is disposed inside the belt guide member 16, and consists of excitation coils 18 and an E shaped magnetic core (core member) 17.
Reference numeral 30 denotes an elastic pressurizing roller which is pressed under a predetermined force to a bottom surface of the belt guide member 16 with the fixing belt 10 interposed so as to form a fixing nip portion N having a predetermined width. The magnetic core 17 of the above described magnetic field producing means 15 is disposed at a location corresponding to the fixing nip portion N.
The pressurizing roller 30 is rotatingly driven by driving means M counterclockwise as indicated by an arrow. When the pressurizing roller 30 is rotatingly driven, a frictional force between the pressurizing roller 30 and an outside surface of the fixing belt 10 exerts a rotating force to the fixing belt 10 in the fixing nip portion N, whereby the fixing belt 10 is set in a condition where it rotates clockwise along an outer circumference of the belt guide member 16 at a circumferential speed nearly corresponding to a circumferential rotating speed of the pressurizing roller 30 while sliding in close contact with a bottom surface of the belt guide member 16 in the fixing nip portion N (pressurizing roller driving type).
The belt guide member 16 functions to apply a pressure to the fixing nip portion N, support the excitation coil 18 and the magnetic core 17 which are used as the magnetic field producing means 15 and the fixing belt 10, and stabilize carriage of the fixing belt 10 during its rotation. The belt guide member 16 is made of an insulating material which does not hinder transmission of a magnetic flux and is bearable of a heavy load.
The excitation coil 18 produces an alternating magnetic flux with an alternating current supplied from an excitation circuit (not shown). The alternating magnetic flux is distributed concentratedly on the fixing nip portion N by the E shaped magnetic core 17 corresponding to the location of the fixing nip portion N and produces an eddy current in the electromagnetic induction heat generating layers of the fixing belt 10 in the fixing nip portion N. This eddy current generates Joule heat by a resistivity of the electromagnetic induction heat generating layers.
This electromagnetic induction heat of the fixing belt 10 is generated concentratedly in the fixing nip portion N in which the alternating magnetic flux is distributed concentratedly and heats the fixing nip portion N with a high efficiency.
A temperature in the fixing nip portion N is controlled so as to be kept at a predetermined level with a temperature control system including temperature detecting means (not shown) which controls a current supplied to the excitation coil 18.
In such a condition where the pressurizing roller 30 is rotatingly driven, the endless fixing belt 10 is rotating along the outer circumference of the belt guide member 16, the fixing belt 10 generates the electromagnetic induction heat with the current supplied from the excitation circuit to the excitation coil 18 and the fixing nip portion N is heated and controlled to the predetermined temperature, a recording material P which is carried from image forming means (not shown) and on which an unfixed toner image t is formed is introduced between the fixing belt 10 and the pressurizing roller 30 in the fixing nip portion N with an image surface set upside, that is, opposed to a surface of the fixing belt, and sandwiched and carried together with the fixing belt 10 through the fixing nip portion N with the image surface kept in close contact with an outside surface of the fixing belt 10 in the fixing nip portion N. At a process where the recording material P is sandwiched and carried together with the fixing belt 10 through the fixing nip portion N, the unfixed toner image on the recording material P is heated and fixed by the electromagnetic induction heat of the fixing belt 10. Upon having passed through the fixing nip portion N, the recording material P is separated from the outside surface of the rotating fixing belt 10, thereafter being discharged and carried.
In a case where the endless fixing belt 10 is driven with the pressurizing roller 30 as in the pressurizing roller driving type fixing apparatus as in the example described above, the recording material P may slip and cannot be carried smoothly when slide resistance between the fixing belt 10 and the belt guide member 16 is higher than the frictional resistance between the pressurizing roller 30 and the fixing belt 10 or when frictional resistance between the pressuring roller 30 and the recording material P is lower than the slide resistance between the fixing belt 10 and the belt guide member 16.
Furthermore, there is a problem that thermal expansion of the elastic layer of the pressurizing roller 30 which functions as a driving roller causes a change of an outside diameter of the pressurizing roller 30, thereby making a carrying speed of the recording material P not constant.
Furthermore, there is another problem that the belt is heavily loaded and is liable to be deteriorated since the belt is driven only at a location which is in slide contact with the guide member.
An object of the present invention is to provide an image heating apparatus which lessens deterioration of a belt to be caused due to driving.
Another object of the present invention is to provide an image heating apparatus which prevents a belt from slipping.
Still another object of the present invention is to provide an image heating apparatus which carries a belt stably.
Still another object of the present invention is to provide an image heating apparatus comprising a movable endless belt, a support member for supporting the belt at an inside of the belt, a holding member for holding the belt at an inside of the belt and being movable together with the belt, and a driving member opposing to the holding member via the belt and driving the belt, wherein the belt slides relative to the support member and an image on a recording material is heated with heat from the belt.
Further another object of the present invention will be apparent from description that follows.